Components and Performance of an Urban Transit System

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The above figure represents an hypothetical urban transit system. Each of its components is designed to provide a specific array of services. Among the defining factors of urban transit are capacity, frequency, flexibility, costs and distance between stops:

• Metro system. Is a heavy rail system, often underground in central areas (parts above ground at more peripheral locations), with fixed routes, services and stations.
  
• Bus system. Is characterized by scheduled fixed routes and stops serviced by motorized multiple passengers vehicles (45 - 80 passengers). Express services, using only a limited number of stops can also be available, notably during peak hours.
• Transit rail system. Fixed rail comes into two major categories. The first is the tram rail system, which is mainly composed of streetcars (tramways) operating in central areas. They can be composed of up to 4 cars. The second is the commuter rail system, which are passenger trains mainly developed to service peripheral / suburban areas through a heavy (faster and longer distances between stations) or light rail system (slower and shorter distances between stations).
  
• Shuttle system. Is composed of a number of privately (dominantly) owned services using small buses or vans. Shuttle routes and frequencies tend to be fixed, but can be adapted to fit new situations. They service numerous specific functions such as expanding mobility along a corridor during peak hour, linking a specific activity center (shopping mall, university campus, industrial zone, hotel, etc.) or aimed at servicing the elderly or people with disabilities.
• Paratransit system. Is a flexible and privately owned collective demand-response system composed of minibuses, vans or shared taxis commonly servicing peripheral and low density zones. Their key advantage is the possibility of a door-to-door service, less loading and unloading time, less stops and more maneuverability in traffic. In many cities of developing countries, this system is informal, dominant and often services central areas because of inadequacies or high costs of the formal transit system.
• Taxi system. Comprises privately owned cars or small vans offering an on-call, individual demand-response system.
  

The table below shows the interval between access points for the public transit modes:

Urban Transit Mode	Distance between stops / stations
Metro	0.5 - 2.0 km
Bus on conventional road network	0.2 - 0.5 km
Bus on reserved lane or express bus	0.8 - 1.6 km
Commuter rail	3.0 - 10.0 km
Light rail	1.0 - 5.0 km

Performance of Urban Transport Modes
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Performance is simply the ratio of speed over space consumption.